The present invention relates to a pneumatic controller according to the preamble of claim 1. Such a pneumatic controller is known from German patent No. 22 11 765. A limit relay is provided in the integral feedback path in order to limit the control pressure to a desired limit pressure. Due to this operation, the following function results: in a batch process (with start-up) the measured value starts from zero in a known manner and accordingly, the control deviation is large and the control pressure larger than the pressure adjusted in the limit relay. During this start-up phase, the limit relay maintains the output pressure at a constant value, so that the controller comprises a fixed feedback, and therefore, a proportional behavior. If the control pressure falls below the pressure adjusted at the limit relay, the limit relay works as a 1:1 transducer, so that a time-delayed feedback, i.e. a controller with PI behavior, is achieved.
This known start-up circuit is relatively simple in design; however, it is only suitable for applications at which the output pressure leaves the control pressure range at its upper limit, since only a maximum limitation is provided. Furthermore, effectiveness of this start-up circuit is not optimum since the controller starts with a characteristic, the working point of which lies at the upper limit of the control pressure range. This leads to the effect that the control value will only leave a fixed value at a point where the measured value attains the desired value. Adjusting the limit relay in such a way that it is actuated by a control pressure which is lower than the maximum control pressure, is not recommended since then the normal PI behavior of the controller within the control range is handicapped.
Departing from this known pneumatic controller, it is, therefore, the object of the present invention to provide this controller with a start-up circuit being simpler and more effective.